Data Analysis: How Has the Lockdown Changed the Pollution Over North India?

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March is the start of spring season in India – when most cities, especially in the north, experience good air quality. A scene near Nizamuddin, New Delhi. Photo: VED/Unsplash.

The year 2020 started off on the wrong foot, thanks to the new coronavirus virtually shutting down the world. India also took some necessary precautions, one of them in the form of a nationwide lockdown that Prime Minister Narendra Modi kicked off on March 24 for 21 days, and which was recently extended to May 3. For a country whose cities are so dense and always teeming with activity, the lockdown has presented sights both eerie and beautiful. In Jalandhar, for example, people could see the Dhauladhar mountain range of Himachal Pradesh, 250 km away, for the first time in 30 years.

People have had to stay indoors, factories have shut and daily-wage labourers have had to reverse-migrate to rural areas. Supply chains have faltered; for the first time in history, the Indian railways stopped operations around the country. This is unprecedented – and an opportunity for us to take a closer look at how air pollution levels have responded to this situation and what we can learn for the future.

March is the start of spring season in India – when most cities, especially in the north, experience good air quality. Just past the cold winter haze, it is a chance to open windows and spend some time in the sun. It is typically welcome relief to citizens who, after many months, don’t have to worry about wearing masks, checking sensors that indicate when it’s safe to go out or huddle around the one purifier in the bedroom.

This spring, thanks to the lockdown, the skies are blue and the air quality index is in code green in most parts of the country. However, it is still no time to step out and enjoy the fresh air. Specifically, the lockdown is an opportunity to understand the extent of air quality changes in various sectors. Cities have very little traffic or commercial and industrial movement. Cooking energy use has also dropped a bit due to interstate mass migrations of workers.

On the first day of the second lockdown (April 15 to May 2), let’s examine some air quality numbers to understand what caused these changes.

The National Capital Region of Delhi, which is the only city with a sizeable number of operational air quality monitors to analyse some data, has six criteria pollutants. Each of the pollutants has a unique story to tell.

PM2.5

PM2.5 is the critical pollutant of Delhi, and the rest of India. During the winter haze, this graph is permanently shaded red or brown, notwithstanding the occasional rain that leads to some sedimentation. Before the lockdown, the high winds of spring had already lowered the PM2.5 levels. After the lockdown began, the lowest daily average was 20 μg/m3, with an average of 35 μg/m3. The main contributor to PM2.5 is combustion. With most local activities at a minimum, what we are seeing is the background baseline concentration.

Is this the lowest we can achieve? For 2017-2019, the monthly average PM2.5 level for March was 100-140 μg/m3. If 35 is the lowest average possible with limited local emissions, without the help of rains, this means at least 70% of pollution is locally generated.

PM10

Resuspended road dust and construction dust are major PM10 contributors. With limited traffic, road dust is under control, and with a complete ban on construction activities, the overall PM10 level has dropped to its lowest day-wise average, at 35 μg/m3. However, dust storms from the west briefly increased the levels since April 10, For 2017-2019, the annual average PM10 level in Delhi was 200 μg/m3.

Sulphur dioxide

For a long time, sulphur dioxide (SO2) was the only pollutant that consistently complied with the national standards. In Delhi, there is no discernible difference between its levels before and after the lockdown. The principal source of SO2 is diesel and coal combustion. Since Delhi already has access to BS-6 fuel, with lowest sulphur content possible, a drop in traffic alone wouldn’t – and expectedly didn’t – affect the data. What we see in the graph is the background contribution to SO2 levels from coal used at power plants, some industries and cooking activities.

Nitrogen dioxide

As the graph indicates, the drop in nitrogen dioxide (NO2) is dramatic enough as well as more discernible compared to the other pollutants. This is because its main source is vehicle exhaust. With nearly 90% of vehicles off the road, the change is evident at ground stations and visible even in satellites’ columnar observations over China, Italy and the USA.

Carbon monoxide

Carbon monoxide (CO) has the longest tropospheric lifetime – approximately two months – of the criteria pollutants, and its levels are incapable of changing quickly. The daytime averages during February-March ranged from 600 to 1000 μg/m3 and during the lockdown period, from 500 to 800 μg/m3.

Ozone

Ozone (O3) is the only secondary pollutant on this list of criteria pollutants. Ozone is formed and destroyed in the presence of nitrogen oxides (NOx) and volatile organic compounds (VOCs). Before the lockdown, the daytime average was in the range of 15-40 μg/m3 and after the lockdown, it is considerably higher, at 25-70 μg/m3.

This is because of chemistry. NOx consists of NO and NO2. NO eats ozone and NO2 makes ozone. Transport is responsible for more than 50% of the NOx emissions in the city. So with a limited amount of NO present, ozone accumulates.

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It would be interesting to examine how the pollution levels change in the absence of lockdown conditions. These counterfactual studies will need some additional modelling based on yet-to-come data from various sectors. The second lockdown period will last until May 3, and we will keep a close eye on the behaviour of the pollutants in the coming weeks.